Which power wattage do I need

go with 600w.

Most 600s would be a little small. You are looking at nearly 400W for just the GPUs, and another 100 or so for the CPU. The good gold 650s can do it, but you might as well get a 750W. That will be fine.

hey 4745454b, do you mind explaining to me what like 80+ bronze or 30+ gold or whatever means? I'm not great with PSUs.

There is no 30+, it's 80+ or nothing.

In the bad old days no one cared about the PSU. We used crap and somehow survived. Then the P4 came out and all of a sudden we had two main things drawing power from the 12V rail. In old PSUs these were nearly an after thought. Even worse, Nvidia and then AMD came out with SLI and then CF and we REALLY needed lots of 12V power. Somebody somewhere decided we should get some sort of ranking for the PSUs so we can tell the ones that can output the power needed for these new types of computers vs those who merely claim they can. At the time most PSUs were ~75% efficient at converting the AC electricity at the wall into DC electricity your PC needs. The rules to get 80+ certified were: must be able to output ALL of the power you claim (so a 500W PSU MUST be able to output all 500W on the rails.), must be at least 80% efficient at converting AC to DC, and must have some form of PFC (PFC removes the little red voltage select switch found on the back of older PSUs.)

While not impossible, if you have an 80+ PSU, it should output all of it's power. Sometimes there are a few companies that claim to be 80+ certified, but are lying about it. There are even a few good PSUs that aren't 80+ certified because they lack PFC. These are usually models that are quality units, but only produced for one region where there is only one "level" of AC to worry about. (USA uses 110V, while Europe uses 230V.) The different levels are 80+, bronze, silver, gold, platinum, titanium. Each level requires the unit to be better at converting AC. The 80+ wiki can give you the output levels and conversion numbers they need to hit. 80+ units at 50% output need to be 80%, bronze at that output I think needs to be 82%. Gold gets up to 87%, while Plat or titanium needs to hit 90%+

The reason you should care about all that is the more efficient at converting a PSU is, the less money it costs you in the long run. If you had one of those horrible older units that only 75% efficient, then to provide 300W to a PC it would be drawing 400W from the wall. A gold unit at 87% however would only draw 345W. Depending on what you pay per kWh, this could be a lot over a month/year. 80+ is also helpful in knowing which PSUs are worth buying. I ignore all units that aren't certified, or only 80+. Bronze is as low as I'm willing to buy. I highly suggest reading the wiki to know more.